Most sites usually have increased activity at certain periods (e.g. 10 pm EST for AcesHigh?), which may give the impression to the user that if he/she stops by at that period there will be some activity.

Almost all the sites rely on real life situations/metaphors to manage the status of their users (e.g. military ranks)

Usually based on experience/past success or reputation points assigned by other members

Some sites rely on random matchmaking based on availability criteria (e.g. yahoo games)

Implementation Possibilities

A relatively simple way to add a calendar to VMT is to install a calendar plugin for Media Wiki.

Some of the scenarios highlighted above would benefit a lot from improved user profiles.

Slightly improved version:

An example profile from the site [Ziizoo]. This is a site that matches tutors to students (i.e. ebay for tutors). They offer chat with whiteboard as two separate applications. They have extensive profiles since that plays a key role in finding a tutor. You can check the functionality they offer at their [blog].